Editing /etc/fstab

You need to know the UUID of your new partition. You can see the mapping by doing:

$ ls -l /dev/disk/by-uuid/

And then add this line to /etc/fstab:

UUID=634c31a5-e27c-4e33-ac67-2e22491a30c2 /usr ext4 defaults 0 2

Change the UUID to your UUID, and change ext4 to be the file system type you are using - you should know this if you have set up the partition.

Delete the old files

After the reboot, the old files in /usr on the root partition will be hidden by the new partition mounted on /usr. But we can use some mount bind trickery to get to the old files and then delete them.

$ sudo mount --bind / /mnt
$ sudo rm -rf /mnt/usr
$ sudo umount /mnt

But some slight mistyping (say, hitting Enter when you'd only typed sudo rm -rf /mnt ) could cause disaster, so I would only use this method if you were very confident in what you were doing, really couldn't deal with any downtime, or had no physical access to the machine and hence were unable to boot off a live CD or live USB stick.

Since most libraries that are used are in /usr, I would not recommend to move this directory while running Ubuntu. In fact, you probably get error messages when you try to do this. Hence, the best is to use the LiveCD.

You can use several possibilities to move/copy the files cp, rsync etc. you want to make sure that any symlinks are created and not just copied. cp and rsync both have options for this.

After moving the files to the other partition you need to add another mount in /etc/fstab to mount the new partition to /usr.